The invention concerns a cryostat having a magnetic coil system including superconducting materials for generation of a magnetic field B0 within a measurement volume, the magnet system having a plurality of radially nested solenoid-shaped coil sections connected in series at least one of which is an LTS section of a conventional low temperature superconductor (LTS) and with at least one HTS section of a high temperature superconductor (HTS), wherein the LTS portion is located in a helium tank of the cryostat having liquid helium at a helium temperature TL<4K.
Cryostats of this kind are e.g. disclosed in DE 10 2004 007 340 A1.
By way of example, nuclear magnetic resonance systems, in particular spectrometers, require very strong, homogenous and stable magnetic fields. The stronger the magnetic field, the better the signal to noise ratio as well as the spectral resolution of the NMR measurement.
Superconducting magnet coil systems are used to produce strong magnetic fields. Magnetic coil systems having solenoid-shaped coil sections are widely used which are nested within each other and operated in series. Superconductors can carry electrical current without losses. The superconducting condition is established below the material-dependent transition temperature. Conventional low temperature superconductors (LTS) are normally utilized for the superconducting material. These metallic alloys, such as NbTi and Nb3S, are relatively easy to process and are reliable in application. An LTS coil-portion conductor usually comprises a normally conducting metallic matrix (e.g. copper) in which superconducting filaments are embedded and which, during normal operation, completely carry the current. In the case of NbTi, these are usually several tens or hundreds of filaments; in the case of Nb3Sn, the filament number could be more than one hundred thousand. Although the internal construction of the conductor is actually somewhat more complex, this is irrelevant within the present context.
The coil sections are cooled with liquid helium within a cryostat in order to cool the superconducting portions below the transition temperature. The superconducting coil sections are thereby at least partially immersed in the liquid helium. For magnets generating the highest fields, the coil sections are, if appropriate, operated using undercooled helium at a temperature beneath 4 K, which even further increases their current carrying capability and their critical magnetic field. The temperature can thereby be at or below the so-called lambda point (approximate 2.2 K) at which the liquid helium becomes superfluid.
In order to even further increase the magnetic field strength of the magnetic coil system it is desirable to also utilize a high temperature superconductor (HTS). For a given temperature, conductors, which include HTS, can carry much more current and thereby achieve higher magnetic field strengths than those with LTS. HTS materials are thereby appropriate for use in the inner most coil sections of a magnetic coil system.
HTS or ceramic superconductors are currently primarily made from bismuth conductors with HTS filaments within a silver matrix. The conductors are usually stripe or band-shaped.
Coil sections made from HTS have turned out to be unreliable and susceptible to short lifetimes, particularly in under-cooled helium. Investigation of defective HTS portions has shown that the HTS material is split open, thereby destroying the current carrying capability of the HTS conductor. This effect, which is also known in other context, is occasionally referred to as “ballooning”.
It is accordingly the purpose of the present invention to present a cryostat in which HTS coil portions enjoy a long lifetime and can be utilized in a reliable manner, in particular, while reducing the risk of ballooning.